Preprint ANL/MCS-P4014-0113The Message Passing Interface (MPI) 3.0 standard includes a significant revision to the remote memory access (RMA) one-sided communication system expected to greatly enhance the usability and performance of MPI RMA. We present the first complete implementation of MPI-3 RMA and document implementation techniques and performance optimization opportunities enabled by the new interface. Our implementation targets messaging-based networks and is publicly available in the latest release of the MPICH MPI implementation. Using this implementation, we explore the performance impact of new MPI-3 functionality and semantics. Results indicate that the MPI-3 RMA interface provides significant advantages over the MPI-2 interface, by enabling significant increase in communication concurrency through relaxed semantics in the interface and additional routines that provide new window types, synchronization modes, and atomic operations.